The present invention relates generally to methods, apparatus and systems for performing laser eye surgery. More particularly, the present invention relates to laser beam position monitoring methods, apparatus and systems for enhancing safety of laser eye surgery systems.
Photorefractive keratectomy (PRK) and phototherapeutic keratectomy (PTK) employ laser beam delivery systems for directing laser energy to a patient's eye to selectively ablate corneal tissue to reform or sculpt the shape of the cornea and, thereby, to improve vision. Present commercial systems often employ excimer lasers. In a first type of system, positioning of the beam is generally fixed and the beam has a cross-sectional area generally corresponding to an entire surface area of a surgical site on the cornea. Cross-sectional portions of the beam are then sequentially masked or adjusted so as to selectively vary the amount of energy exposure of different portions of the surgical site so as to effect the desired sculpting. This can typically be achieved by using an iris or other exposure control mechanism. While highly effective and relatively easy to control, employing a laser beam having a cross-sectional area generally equal to the area of the treatment or surgical site (typically having a diameter of 5.0 mm to 10.0 mm) often involves the use of relatively large amounts of energy. This is typically relatively expensive, and leads to relatively large laser systems.
As an alternative to such large beam diameter systems, laser “scanning” systems can be employed for corneal ablation. Such scanning systems typically employ a laser beam having a smaller cross-sectional area, thereby decreasing energy requirements. Accordingly, laser scanning systems delivering laser beams of relatively small cross-sectional area can be more economic to use and normally are of smaller construction than laser systems having larger diameter beams. However, the use of such small beams complicates certain aspects of the treatment protocols required to perform the sculpting. For example, to achieve a desired level of volumetric tissue removal or ablation from the eye, the treatment beam is scanned over or otherwise moved across the eye from one position to a next during the surgical procedure. Movement of the beam is typically achieved through motorized scanning mechanisms, devices, or the like. These scanning mechanisms often regulate the position of an optical element, such as the angle of a mirrored surface, the lateral position of an offset imaging lens or the like, so as to adjust the lateral position of the beam across the treatment site. In a related type of system, the laser beam is scanned over the corneal surface while varying the cross-section of the laser beam.
To achieve properly controlled laser exposure over the entire treatment site on the eye, the positioning of a scanning laser beam should be controlled accurately. If the beam resides at one position for too long, due to a jam or malfunction of the scanning mechanism for example, the desired tissue ablation pattern may not be achieved. A jam of the scanning system may jeopardize the success of the surgery and could cause damage to the patient's eye. Since the laser beam itself is not easily visible, malfunction of some scanning mechanisms may not be readily detectable by an observer.
Thus, it would be desirable to provide methods, apparatus and/or systems for monitoring a laser beam position in a laser surgery system. It would also be desirable to verify that a laser beam position matches a desired beam position and to stop a laser surgery procedure if the beam position does not match the desired position. Preferably, such methods, apparatus and the like could be incorporated into a laser surgery system in a cost-effective manner without interfering with performance of the system. Though such methods, apparatus or subsystems would find particular use in scanning laser beam systems, they may also feasibly be used with large diameter laser beam systems.